Various implantable medical devices have been developed that receive information from one or more physiologic sensors or transducers. A typical physiologic sensor transduces a measurable parameter of the human body, such as blood pressure, temperature or oxygen saturation for example, into corresponding electrical signals. At the appropriate time, the physiologic data acquired by an implantable medical device is uplinked to an external receiving system, such as a programmer, for storage and analysis.
In many implantable medical device applications, a radio frequency (RF) telemetry approach is used by which data acquired by an implantable medical device is impressed on a carrier signal and transmitted to an external receiving system during an data uplink procedure. A demodulation circuit is typically provided in the receiving system that recovers the physiologic information signal from a modulated signal received from the implantable medical device. An analog low pass filter is typically used to extract the physiologic information signal from the demodulated signal, which generally contains noise and exhibits other undesirable corruptive characteristics.
In many conventional receiving systems, noise is introduced into the RF signal channel from a number of traditional sources which corrupts the demodulated data signal. In addition, high frequency noise components are introduced into the demodulated data signal as a result of the demodulation process. Moreover, receiving systems that employ analog components generally experience problems that normally arise from processing of analog signals with relatively large signal swings and large bandwidths. Such problems include bit distortion due to low-to-high and high-to-low slew rate differences, failure to follow distortion (i.e., DC level shifts) caused by parasitic capacitances and charging of capacitors, failure to follow distortion caused by data in which there may be more positive data than negative data, and inter-symbol interference caused by tradeoffs in analog filter performance.
A traditional approach to addressing such signal corruption and processing problems involves the use of a programmable analog low pass filter, such as use of filters implemented in analog integrated circuits (IC's) and analog switches. Another traditional analog approach involves the use of a switched capacitor filter and an adjustable clock. Although analog low pass filtering provides for some degree of improvement in the extracted data signal, a successful analog low pass filtering circuit implementation is typically complex and requires a significant amount of printed circuit board space.
Further, a traditional analog low pass filtering approach typically involves a "custom" design, which is generally intolerant to changes in data rates and the frequency response of the filter. Such custom designs tend to be both expensive and limited in terms of the potential to use standardized, readily available, and relatively inexpensive electronic components. Still further, the use of analog components in a particular filter design is generally associated with increased power consumption, in contrast to a fully digital implementation. Increasing the power consumption requirements of the receiving system, including the filtering circuitry, may pose a significant problem in portable and small scale receiver applications.
Various implementations of RF telemetry systems directed for use with an implantable medical device are known in the art, examples of which may be found in the issued U.S. Patents listed in Table 1 below.
TABLE 1 U.S. Pat. No. Inventor(s) Issue Date 4,281,664 Duggan August 4, 1981 4,494,545 Slocum et al. January 22, 1985 4,556,063 Thompson et al. December 3, 1985 4,562,840 Batina et al. January 7, 1986 4,571,589 Slocum et al. February 18, 1986 4,681,111 Silvian July 21, 1987 4,757,816 Ryan et al. July 19, 1988 4,949,299 Silvian July 31, 1990 5,058,581 Silvian October 22, 1991 5,107,833 Barsness April 28, 1992 5,127,404 Wyborny at al. July 7, 1992 5,241,961 Henry September 7, 1993 5,264,843 Silvian November 23, 1993 5,292,343 Blanchette et al. March 8, 1994 5,300,093 Koestner et al. April 5, 1994 5,312,453 Shelton et al. May 17, 1994 5,383,912 Cox et al. January 24, 1995 5,475,307 Silvian December 12, 1995 5,620,472 Rahbar April 15, 1997
The patents listed in Table 1 hereinabove are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of Various Embodiments, and the claims set forth below, many of the devices and methods disclosed in the patents identified below and listed in Table 1 above may be modified advantageously by using the teachings of the present invention.